Chapter 6
Processes to Significantly Reduce Pathogens (PSRPs)
6.1 Introduction
Processes to Significantly Reduce Pathogens (PSRPs)
are listed in Appendix B of Part 503. There are five PSRPs:
aerobic and anaerobic digestion, air drying, composting,
and lime stabilization. Under Part 503.32(b)(3), sewage
sludge meeting the requirements of these processes is
considered to be Class B with respect to pathogens (see
Section 5.3). When operated under the conditions speci­
fied in Appendix B, PSRPs reduce fecal coliform densities
to less than 2 million CFU or MPN per gram of total solids
(dry weight basis) and reduce Salmonella sp. and enteric
virus densities in sewage sludge by approximately a fac­
tor of 10 (Farrell, et al., 1985).
This level of pathogen reduction is required, as a mini­
mum, by the Part 503 regulation if the sewage sludge is
applied to agricultural land, a public contact site, a forest,
or a reclamation site or placed on a surface disposal site1.
Because Class B biosolids may contain some pathogens,
land application of Class B biosolids is allowed only if crop
harvesting, animal grazing, and public access are limited
for specific periods of time following application of Class B
biosolids so that pathogens can be further reduced by en­
vironmental factors (see Section 5.5).
The PSRPs listed in Part 503 are essentially identical to
the PSRPs that were listed under the 40 CFR Part 257
regulation, except that all requirements related solely to
reduction of vector attraction have been removed. Vector
attraction reduction is now covered under separate require­
ments (see Chapter 8) that include some of the require­
ments that were part of the PSRP requirements under Part
257, as well as some new options for demonstrating vec­
tor attraction reduction. These new options provide greater
flexibility to the regulated community in meeting the vector
attraction reduction requirements.
Although theoretically two or more PSRP processes,
each of which fails to meet its specified requirements, could
be combined and effectively reduce pathogens (i.e. partial
treatment in digestion followed by partial treatment by air
drying) it cannot be assumed that the pathogen reduction
contribution of each of the operations will result in the 2­
log reduction in fecal coliform necessary to define the com­
bination as a PSRP. Therefore, to comply with Class B
pathogen requirements, one of the PSRP processes must
be conducted as outlined in this chapter, or fecal coliform
testing must be conducted in compliance with Class B Al­
ternative 1. The biosolids preparer also has the option of
applying for PSRP equivalency for the combination of pro­
cesses. Achieving PSRP equivalency enables the preparer
to stop monitoring for fecal coliform density.
This chapter provides detailed descriptions of the PSRPs
listed in Appendix B. Since the conditions for the PSRPs,
particularly aerobic and anaerobic digestion, are designed
to meet pathogen reduction requirements, they are not
necessarily the same conditions as those traditionally rec­
ommended by environmental engineering texts and manu­
als.
6.2 Aerobic Digestion
In aerobic digestion, sewage sludge is biochemically
oxidized by bacteria in an open or enclosed vessel (see
photo). To supply these aerobic microorganisms with
enough oxygen, either the sewage sludge must be agi­
tated by a mixer, or air must be forcibly injected (Figure 6­
1). Under proper operating conditions, the volatile solids
in sewage sludge are converted to carbon dioxide, water,
and nitrate nitrogen.
Aerobic systems operate in either batch or continuous
mode. In batch mode, the tank is filled with untreated sew­
age sludge and aerated for 2 to 3 weeks or longer, de­
pending on the type of sewage sludge, ambient tempera­
ture, and average oxygen levels. Following aeration, the
stabilized solids are allowed to settle and are then sepa­
rated from the clarified supernatant. The process is begun
again by inoculating a new batch of untreated sewage
sludge with some of the solids from the previous batch to
supply the necessary biological decomposers. In continu­
ous mode, untreated sewage sludge is fed into the digester
once a day or more frequently; thickened, clarified solids
are removed at the same rate.
The PSRP description in Part 503 for aerobic digestion
is:
• Sewage sludge is agitated with air or oxygen to maintain aerobic conditions for a specific mean cell resi­
1Unless
the active biosolids surface disposal unit is covered at the end of each
operating day, in which case no pathogen requirement applies.
43
Digester in Vancouver, Washington.
Settling
Tank
Aerodigester
Raw
Sludge
Oxidized
Overflow
to Treatment Works
Return Sludge
to Aerodigester
Figure 6-1. Aerobic digestion.
44
dence time at a specific temperature. Values for the
mean cell residence time and temperature shall be
between 40 days at 20°C (68°F) and 60 days at 15°C
(59°F).
For temperatures between 15°C (59°F) and 20°C (68°F)
use the relationship between time and temperature pro­
vided below to determine the required mean cell residence
time.
Time @T°C = 1.08 (20-T)
40 d
The regulation does not differentiate between batch, in­
termittently fed, and continuous operation, so any method
is acceptable. The mean cell residence time is considered
the residence time of the sewage sludge solids. The ap­
propriate method for calculating residence time depends
on the type of digester operation used (see Appendix E).
Continuous-Mode, No Supernatant Removal For continuous-mode digesters where no supernatant is removed,
nominal residence times may be calculated by dividing liq­
uid volume in the digester by the average daily flow rate in
or out of the digester.
Continuous-Mode, Supernatant Removal In systems
where the supernatant is removed from the digester and
recycled, the output volume of sewage sludge can be much
less than the input volume of sewage sludge. For these
systems, the flow rate of the sewage sludge out of the
digester is used to calculate residence times.
Continuous-Mode Feeding, Batch Removal of Sew­
age Sludge For some aerobic systems, the digester is
initially filled above the diffusers with treated effluent, and
sewage sludge is wasted daily into the digester. Periodi­
cally, aeration is stopped to allow solids to settle and su­
pernatant to be removed. As the supernatant is drawn off,
the solids content in the digester gradually increases. The
process is complete when either settling or supernatant
removal is inadequate to provide space for the daily sew­
age sludge wasting requirement, or sufficient time for di­
gestion has been provided. The batch of digested sewage
sludge is then removed and the process begun again. If
the daily mass of sewage sludge solids introduced has
been constant, nominal residence time is one-half the to­
tal time from initial charge to final withdrawal of the digested
sewage sludge.
Batch or Staged Reactor Mode A batch reactor or two
or more completely-mixed reactors in series are more ef­
fective in reducing pathogens than is a single well-mixed
reactor at the same overall residence time. The residence
time required for this type of system to meet pathogen re­
duction goals may be 30% lower than the residence time
required in the PSRP definition for aerobic digestion (see
Appendix E). However, since lower residence times would
not comply with PSRP conditions required for aerobic di­
gestion in the regulation, approval of the process as a PSRP
by the permitting authority would be required.
Other Digesters are frequently operated in unique ways
that do not fall into the categories above. Appendix E pro­
vides information that should be helpful in developing a
calculation procedure for these cases. Aerobic digestion
carried out according to the Part 503 requirements typi­
cally reduces bacterial organisms by 2-log and viral patho­
gens by 1-log. Helminth ova are reduced to varying de­
grees, depending on the hardiness of the individual spe­
cies. Aerobic digestion typically reduces the volatile solids
content (the microbes’ food source) of the sewage sludge
by 40% to 50%, depending on the conditions maintained
in the system.
Vector Attraction Reduction
Vector attraction reduction for aerobically digested sew­
age sludges is demonstrated either when the percent vola­
tile solids reduction during sewage sludge treatment equals
or exceeds 38%, or when the specific oxygen uptake rate
(SOUR) at 20°C (68°F) is less than or equal to 1.5 mg of
oxygen per hour per gram of total solids, or when addi­
tional volatile solids reduction during bench-scale aerobic
batch digestion for 30 additional days at 20°C (68°F) is
less than 15% (see Chapter 8).
Thermophilic aerobic systems (operating at higher tem­
peratures) capable of producing Class A biosolids are de­
scribed in Section 7.5.
6.3 Anaerobic Digestion
Anaerobic digestion is a biological process that uses
bacteria that function in an oxygen-free environment to
convert volatile solids into carbon dioxide, methane, and
ammonia. These reactions take place in an enclosed tank
(see Figure 6-2) that may or may not be heated. Because
the biological activity consumes most of the volatile solids
needed for further bacterial growth, microbial activity in
the treated sewage sludge is limited. Currently, anaerobic
digestion is one of the most widely used treatments for
sewage sludge treatment, especially in treatment works
with average wastewater flow rates greater than 19,000
cubic meters/day (5 million gallons per day).
Most anaerobic digestion systems are classified as ei­
ther standard-rate or high-rate systems. Standard-rate
systems take place in a simple storage tank with sewage
sludge added intermittently. The only agitation that occurs
comes from the natural mixing caused by sewage sludge
gases rising to the surface. Standard-rate operation can
be carried out at ambient temperature, though heat is some­
times added to speed the biological activity.
High-rate systems use a combination of active mixing
and carefully controlled, elevated temperature to increase
the rate of volatile solids destruction. These systems some­
times use pre-thickened sewage sludge introduced at a
uniform rate to maintain constant conditions in the reactor.
Operating conditions in high-rate systems foster more effi­
cient sewage sludge digestion.
The PSRP description in Part 503 for anaerobic diges­
tion is:
45
Second Stage
(stratified)
First Stage
(completely mixed)
Figure 6-2. Two-stage anaerobic digestion (high rate).
• Sewage sludge is treated in the absence of air for a
specific mean cell residence time at a specified tem­
perature. Values for the mean cell residence time and
temperature shall be between 15 days at 35°C to 55°C
(95°F to 131°F) and 60 days at 20°C (68°F).
Straight-line interpolation to calculate mean cell residence time is allowable when the temperature falls between 35°C and 20°C.
Section 6.2 provides information on calculating residence
times. Anaerobic digestion that meets the required resi­
dence times and temperatures typically reduces bacterial
and viral pathogens by 90% or more. Viable helminth ova
are not substantially reduced under mesophilic conditions
(32°C to 38°C [90°F to 100°F]) and may not be completely
reduced at temperatures between 38°C (100°F) and 50°C
(122°F).
is usually applied to a depth of approximately 23 cm (9
inches) onto sand drying beds, or even deeper on paved
or unpaved basins. The sewage sludge is left to drain and
dry by evaporation. Sand beds have an underlying drain­
age system; some type of mechanical mixing or turning is
frequently added to paved or unpaved basins. The effec­
tiveness of the air drying process depends very much on
the local climate: drying occurs faster and more completely
in warm, dry weather, and slower and less completely in
cold, wet weather. During the drying/storage period in the
bed, the sewage sludge is undergoing physical, chemical,
and biological changes. These include biological decom­
position of organic material, ammonia production, and des­
iccation.
Anaerobic systems reduce volatile solids by 35% to 60%,
depending on the nature of the sewage sludge and the
system’s operating conditions. Sewage sludges produced
by systems that meet the operating conditions specified
under Part 503 will typically have volatile solids reduced
by at least 38%, which satisfies vector attraction reduction
requirements. Alternatively, vector attraction reduction can
be demonstrated by Option 2 of the vector attraction re­
duction requirements, which requires that additional vola­
tile solids loss during bench-scale anaerobic batch diges­
tion of the sewage sludge for 40 additional days at 30°C to
37°C (86°F to 99°F) be less than 17% (see Section 8.3).
The SOUR test is an aerobic test and cannot be used for
anaerobically digested sewage sludge.
6.4 Air Drying
Air drying allows partially digested sewage sludge to dry
naturally in the open air (see photo). Wet sewage sludge
Sludge drying operation. (Photo credit: East Bay Municipal Utility
District)
46
The PSRP description in Part 503 for air drying is:
Vector Attraction Reduction
• Sewage sludge is dried on sand beds or on paved or
unpaved basins. The sewage sludge dries for a mini­
mum of 3 months. During 2 of the 3 months, the ambi­
ent average daily temperature is above 0°C (32°F).
Air-dried sewage sludge typically is treated by aerobic
or anaerobic digestion before it is placed on drying beds.
Usually, the easiest vector attraction reduction requirement
to meet is a demonstration of 38% reduction in volatile
solids (Option 1, See Section 8.2), including the reduction
that occurs during its residence on the drying beds.
Although not required by the Part 503, it is advisable to
ensure that the sewage sludge drying beds are exposed
to the atmosphere (i.e., not covered with snow) during the
2 months that the daily temperature is above 0°C (32°F).
Also, the sewage sludge should be at least partially di­
gested before air drying. Under these conditions, air dry­
ing will reduce the density of pathogenic viruses by 1-log
and bacteria by approximately 2-log. Viable helminth ova
also are reduced, except for some hardy species that re­
main substantially unaffected.
In dry climates, vector attraction reduction can be
achieved by moisture reduction (see Option 7 in Section
8.8, and Option 8 in Section 8.9).
6.5 Composting
Composting involves the aerobic decomposition of or­
ganic material using controlled temperature, moisture, and
oxygen levels. Several different composting methods are
currently in use in the United States. The three most com­
mon are windrow, aerated static pile, and within-vessel
composting. These are described below.
Vector Attraction Reduction
Frequently sand-bed drying follows an aerobic or anaero­
bic digestion process that does not meet the specified pro­
cess requirements and does not produce 38% volatile sol­
ids destruction. However, it may be that the volatile solids
reduction produced by the sequential steps of digestion
and drying will meet the vector attraction reduction require­
ment of 38% volatile solids reduction. If this is the case,
vector attraction reduction requirements are satisfied.
Example of Meeting PSRP and
Vector Attraction Reduction Requirements
Air Drying
Type of Facility
B
Class
Pathogen Reduction Partially digested sewage
sludge is thickened and
spread in drying beds. Filling
of beds starts in June, and the
beds accommodate sewage
sludge generated over 1 full
year. Beds are then emptied
the following September so
that all sewage sludge is re­
tained over an entire summer
(>0°C ambient temperatures).
Vector Attraction
Reduction
Sewage sludge is tested for
pollutants 2 weeks before
material is removed and dis­
tributed.
Biosolids are land applied
and plowed immediately into
Use or Disposal
the soil.
Biosolids are delivered to lo­
Testing
cal farmers. Farmers are
given information on site re­
strictions, and must follow
harvest, grazing, and public
access restrictions.
Composting can yield either Class A or Class B biosolids,
depending on the time and temperature variables involved
in the operation.
All composting methods rely on the same basic pro­
cesses. Bulking agents such as wood chips, bark, saw­
dust, straw, rice hulls, or even-finished compost are added
to the sewage sludge to absorb moisture, increase poros­
ity, and add a source of carbon. This mixture is stored (in
windrows, static piles, or enclosed tanks) for a period of
intensive decomposition, during which temperatures can
rise well above 55°C (131°F). Depending on ambient tem­
peratures and the process chosen, the time required to
reduce pathogens and produce Class B biosolids can range
from 3 to 4 weeks. Aeration and/or frequent mixing or turn­
ing are needed to supply oxygen and remove excess heat.
Following this active stage, bulking agents may or may
not be screened from the completed compost for recycling
(see photo), and the composted biosolids are “cured” for
an additional period.
Windrow composting involves stacking the sewage
sludge/bulking agent mixture into long piles, or windrows,
generally 1.5 to 2.7 meters high (5 to 9 feet) and 2.7 to 6.1
meters wide (9 to 20 feet). These rows are regularly turned
or mixed with a turning machine or front-end loader to fluff
up the material and increase porosity which allows better
convective oxygen flow into the material. Turning also
breaks up compacted material and reduces the moisture
content of the composting media (see photo, next page).
Active windrows are typically placed in the open air, ex­
cept in areas with heavy rainfall. In colder climates, winter
weather can significantly increase the amount of time
needed to attain temperatures needed for pathogen re­
duction.
Aerated static pile composting uses forced-air rather than
mechanical mixing (see Figure 6-3) to both supply suffi­
cient oxygen for decomposition and carry off moisture. The
sewage sludge/bulking agent mixture is placed on top of
47
Agitated bed systems (one type of within-vessel
composting) depend on continuous or periodic mixing
within the vessel, followed by a curing period.
Pathogen reduction during composting depends on time
and temperature variables (see photo page 49). Part 503
provides the following definition of PSRP requirement for
pathogen reduction during composting:
Composted sludge is screened to remove the bulking agent prior
to land application
• Using either the within-vessel, static aerated pile, or
windrow composting methods, the temperature of the
sewage sludge is raised to 40°C (104°F) or higher and
remains at 40°C (104°F) or higher for 5 days. For 4
hours during the 5-day period, the temperature in the
compost pile exceeds 55°C (131°F).
These conditions, achieved using either within-vessel,
aerated static pile, or windrow methods, reduce bacterial
pathogens by 2-log and viral pathogens by 1-log.
A process time of only 5 days is not long enough to fully
break down the volatile solids in sewage sludge, so the
composted sewage sludge produced under these condi­
tions will not be able to meet any of the requirements for
reduced vector attraction. In addition, sewage sludge that
has been composted for only 5 days may still be odorous.
Breakdown of volatile solids may require 14 to 21 days for
within-vessel; 21 or more days for aerated static pile; and
30 or more days for windrow composting. Many treatment
works allow the finished sewage sludge compost to fur­
ther mature or cure for at least several weeks following
active composting during which time pile turning or active
aeration may continue.
Composting is most often used to meet Class A require­
ments. More guidance for composting operations and how
to meet Class A time and temperature requirements is pro­
vided in Chapter 7.
Vector Attraction Reduction
Compost mixing equipment turns over a windrow of compost for
solar drying prior to screening [Photo credit: East Bay Municipal
Utility District)
either (1) a fixed underlying forced aeration system, or (2)
a system of perforated piping laid on the composting pad
surface and topped with a bed of bulking agent. The entire
pile is covered with a layer of cured compost for insulation
and odor control. Pumps are used to blow air into the com­
post pile or suck air through it. The latter provides greater
odor control because the compost air can be easily col­
lected and then filtered or scrubbed.
Within-vessel composting systems vary greatly in de­
sign, but they share two basic techniques: the process
takes place in a reactor vessel where the operating condi­
tions can be carefully controlled (see photo page 49), and
active aeration meets the system’s high oxygen demand.
Vector attraction reduction must be conducted in accor­
dance with Option 5, or compost must be incorporated into
soil when land applied. This option requires aerobic treat­
ment (i.e., composting) of the sewage sludge for at least
14 days at over 40°C (104°F) with an average tempera­
ture of over 45°C (113°F).
6.6 Lime Stabilization
The lime stabilization process is relatively straightforward:
lime - either hydrated lime, Ca(OH)2; quicklime, CaO; or
lime containing kiln dust or fly ash - is added to sewage
sludge in sufficient quantities to raise the pH above 12 for
2 hours or more after contact, as specified in the Part 503
PSRP description for lime stabilization:
• Sufficient lime is added to the sewage sludge to raise
the pH of the sewage sludge to 12 after 2 hours of
contact.
For the Class B lime stabilization process, the alkaline
material must be a form of lime. Use of other alkaline ma­
48
Taulman Weiss in-vessel composting facility in Portland, Oregon.
Compost operator measures compost pile temperature as part of
process monitorlng. (Photo credit: East Bay Municipal Utility District,
Oakland, California)
Wood Chips or
Compost
Filter Pile of
Composted Sludge
Figure 6-3. Static aerated pile composting.
terials must first be demonstrated to be equivalent to a
PSRP. Elevation of pH to 12 for 2 hours is expected to
reduce bacterial and viral density effectively.
Lime may be introduced to liquid sewage sludge in a
mixing tank or combined with dewatered sewage sludge,
providing the mixing is complete and the sewage sludge
cake is moist enough to allow aqueous contact between
the sewage sludge and lime.
Mixing must be sufficient to ensure that the entire mass
of sewage sludge comes into contact with the lime and
undergoes the increase in pH and to ensure that samples
are representative of the overall mixture (see Chapter 9).
pH should be measured at several locations to ensure that
the pH is raised throughout the sewage sludge.
A variety of lime stabilization processes are currently in
use. The effectiveness of any lime stabilization process
for controlling pathogens depends on maintaining the pH
at levels that reduce microorganisms in the sewage sludge.
Field experience has shown that the application of lime
stablized material after the pH has dropped below 10.5
may, in some cases, create odor problems. Therefore it is
49
recommended that biosolids application take place while
the pH remains elevated. If this is not possible, and odor
problems develop, alternate management practices in the
field include injection or incorporation or top dressing the
applied biosolids with additional lime. Alternate manage­
ment practices if the biosolids have not yet left the waste­
water treatment plant may include adding additional lime
to maintain the elevated pH or additional treatment through
drying or composting. Lime stabilization can reduce bac­
terial and viral pathogens by 99% or more. Such alkaline
conditions have little effect on hardy species of helminth
ova, however.
Vector Attraction Reduction
For lime-treated sewage sludge, vector attraction reduc­
tion is best demonstrated by Option 6 of the vector attrac­
tion reduction requirements. This option requires that the
sewage sludge pH remain at 12 or higher for at least 2
hours, and then at 11.5 or more for an additional 22 hours
(see Section 8.7).
Lime stabilization does not reduce volatile solids. Field
experience has shown that the application of lime stabi­
lized material after the pH has dropped below 10.5 may
create odor problems. Therefore it is recommended that
land application of biosolids take place as soon as pos­
sible after vector attraction reduction is completed and while
pH remains elevated.
6.7 Equivalent Processes
Table 11.1 in Chapter 11 lists some of the processes
that the EPA’s Pathogen Equivalency Committee has rec­
ommended as being equivalent to PSRP under Part 257.
Information on the PEC and how to apply for equivalency
are discussed in Chapter 11.
References and Other Resources
Berg, G. and D. Berman. 1980. Destruction by anaero­
bic mesophilic and thermophilic digestion of vi­
ruses and indicator bacteria indigenous to do­
mestic sludges. Appl. Envir. Microbiol. 39(2):361368.
Farrell, J.B., G. Stern, and A.D. Venosa. 1990. Mi­
crobial destructions achieved by full-scale
anaerobic digestion. Paper presented at Munici­
pal Wastewater Sludge Disinfection Workshop.
Kansas City, MO. Water Pollution Control Fed­
eration, October 1995.
U.S. EPA. 1992. Technical support document for reduction of pathogens and vector attraction in sew­
age sludge. EPA/822/R-93/004.
50